Use of deuterium oxide-depleted water as a tracer in downhole and core analysis applications

ABSTRACT

A method of ascertaining or measuring subterranean events includes use of deuterium oxide-depleted water as a tracer. The presence of deuterium oxide-depleted water may be measured at a subterranean location by means such as core sampling, for any purpose for which tracers are known to be useful. For example, the presence or absence, and level, of deuterium oxide-depleted water in a core sample may be used to determine fluids saturation levels, porosity and permeability, as well as other subterranean formation and/or reservoir characteristics important to the oil and gas production industry.

BACKGROUND

1. Field of the Invention

This invention relates to the field of tracers used in a variety ofapplications, and in particular to methods of ascertaining or measuringsubterranean events wherein tracer migration may occur.

2. Background Art

The use of so-called “tracers” in a variety of applications is wellknown. “Tracers” represent a variety of chemical compounds that, underthe conditions of use, act or react in a predictable way such that theirpresence, absence or activity in a particular location may be detected.This detection is useful in ascertaining and/or measuring eventsoccurring at that location, i.e., in obtaining information about aparticular microcosm such as, for example, a particular downhole orformation region of a drilling site.

Tracers that have traditionally been used in oil and gas drillingapplications include, for example, salts of various types, such aspotassium chloride, in solution; inert gases, such as krypton or xenon;and various hydrocarbon compounds. These materials operate to “tag”,i.e., act as a tracer in, oil, gas, steam or water which is introducedinto the subterranean formation or well-bore using any of a wide varietyof known equipment and/or methods. Other materials that may be used astracers are so-called “heavy water”, which is deuterium oxide, andtritium. Both deuterium oxide and tritium are effective for taggingpurposes, but each suffers from some drawback. For example, deuteriumoxide is relatively expensive because of the ratio of water required toform the desired product (about 10 tons water to prepare 1 kg deuteriumoxide). Furthermore, while deuterium oxide can be purchased inrelatively pure form, it is subject to very strict import and/or exportrestrictions. In contrast, tritium is somewhat less expensive and lessrestricted than deuterium oxide. However, tritium is still moreexpensive than many other tracers and may be undesirable or lessdesirable for certain applications.

Thus, what is needed in the art is a tracer material which is effectivein applications where it will be employed, and which represents aneconomically feasible and acceptably safe material.

SUMMARY

An object of the invention is to provide a tracer material which may beeffectively used in subterranean applications.

Another object of the invention is to provide a tracer material whichmay be used in subterranean or surface applications within parameters ofacceptable expense and safety.

In carrying out these and other objects of the invention, there isprovided, in one aspect, a method of ascertaining or measuring an eventcomprising using deuterium oxide-depleted water as a tracer.

In another aspect, there is provided a method of ascertaining ormeasuring a subterranean event comprising transporting deuteriumoxide-depleted water, as a tracer, to a first subterranean location. Thetracer is given a time period sufficient for it to migrate to a secondsubterranean location, if such migration is possible. A tracer detectionmeans or method is then employed to detect the tracer in the secondsubterranean location.

In still another aspect, there is provided a method of ascertaining ormeasuring filtrate contamination of a subterranean formation. Thismethod comprises transporting deuterium oxide-depleted water, as atracer, to a downhole location. This downhole location has, adjacent toit, a subterranean formation wherein fluid is located. A time isallowed, sufficient for the tracer to infiltrate the adjacentsubterranean formation, assuming such infiltration is possible. A sampleof the subterranean formation is then obtained, such that the fluidtherein is preserved. Finally, the amount of tracer in the fluid ismeasured.

DETAILED DESCRIPTION

The invention's method involves use of deuterium oxide-depleted water asa tracer. Deuterium oxide is itself the entity that has beencolloquially termed “heavy water”. It is, in its relatively pure form, acompound formed of three atoms, in which one of the two hydrogen atomscontains one extra neutron, i.e., it is HD0 rather than H₂ 0. Deuteriumthus can be distinguished from hydrogen by virtue of its higher atomicmass. Deuterium oxide is naturally occurring, and is virtually alwayspresent in natural waterways, including both surface and subterraneanwaters, in amounts frequently ranging from about 100 to about 200 ppm,generally from about 120 to about 180 ppm, and most frequently fromabout 130 to about 160 ppm.

Various methods have been developed to prepare so-called heavy water.For example, distillation, condensation and electrolysis means may beemployed for this purpose, resulting in concentration of deuterium oxidein ordinary water. A by-product of some of these methods is deuteriumoxide-depleted water. For convenience hereinafter, this deuteriumoxide-depleted material will be referred to as “light water”, incontrast with deuterium oxide-rich “heavy water”. Light water iscommercially available having deuterium present in amounts of only about1 to about 3 ppm, but for purposes herein and in some non-limitingembodiments, the term “light water” will be used to refer to water inwhich deuterium levels are anywhere from about 0 ppm to about 50 ppm,and in other non-limiting embodiments from about 0 ppm to about 25 ppm.

The advantage of using light water in tracer applications, particularlyfor downhole purposes, is that the light water may be distinguished fromother water, such as water typically present in formations or frommunicipal water sources, by the light water's relatively reduced levelsof deuterium or deuterium oxide. While light water is generally moreexpensive than such formation or municipial water, its cost is generallyacceptable for oil industry tracer purposes and it is generallyconsidered to be relatively safe. It is therefore also generally free ofexport, import or usage restrictions in most, if not all, jurisdictions.

Since deuterium and/or deuterium oxide can be both detected andquantified using a variety of methods, measurements thereof, taken in orfrom samples obtained from a subterranean formation may be used todetermine whether the formation has been infiltrated by the deuteriumoxide-depleted water. The deuterium or deuterium oxide may be detectedby, for example, methods such as conventional mass spectrophotometry,gas chromatograph-mass spectrophotometry, HPLC mass spectrophotometry,and nuclear magnetic resonance (NMR) imaging, including but not limitedto both downhole and surface use thereof. Those skilled in the art willbe aware of procedures for appropriate sample obtention and testing ofsuch samples for analysis by any or all of the above means.

In the method of the invention a tracer detection means or method isemployed. In the case of a tracer detection means, which may be selectedto be specific to the means that will be employed for analysis, it maybe generally described as any apparatus that initiates a detectionsignal upon locating an amount of deuterium or deuterium oxide. Thissignal predictably varies in some way, according to the amount ofdeuterium or deuterium oxide located. This signal is then transferred toan associated detection signal receiving and interpreting means (whichmay be a single means or two or more separate means), which provides theinformation to the user in an appropriate form. Such form may be, innon-limiting embodiments, a read-out means, such as a meter or graphicmeans, and/or another type of means, such as a means for directly orindirectly modifying events relating to the location of the originaldetection signal or another location, such as the location from which aspecific coring sample was obtained. In the case of a tracer detectionmethod, it may generally be described as any procedure, or series ofprocedures or protocols, that may serve to detect and, in some cases, toquantify the tracer. These may include a variety of combinations ofsampling, preservation, extraction, isolation, analysis, and/oridentifying operations. In many embodiments both a tracer detectionmeans and a tracer detection method may be effectively used.

Such tracer detection means or method may be employed in variousapplications such as conventional drilling core analysis, in whichextracted core samples are evaluated. Such evaluation may be carried outto determine various properties of the formation and/or reservoir, suchas porosity, permeability, fluid saturation, and so forth. The inventionmay also be employed in cases of, in one embodiment, sponge coring, inwhich a cylindrical core is cut from a subterranean formation andtransported to the surface for analysis. In this case, an absorbent,i.e., sponge-like, material, hereinafter termed the “sponge”, is used toline the core barrel assembly and absorb any fluid to which it isexposed. This is particularly useful where the ambient atmosphericpressure to which the core sample is exposed upon extraction to thesurface is significantly less than the downhole pressure. The differencein pressure causes the formation's gases within the core sample toexpand and be expelled from the core as it rises to the surface, whilethe generally annular sponge absorbs and preserves the contacted fluidfor later analysis. The fluid extracted may be formation fluid, drillingmud fluid, or a combination thereof, depending on factors inherent inthe fluid's location in the core sample, the properties of theformation, and the obtention and processing of the core sample.

In such embodiment the light water tracer may be introduced into theformation, using any known method and/or equipment, at a predeterminedpoint. Such is most conveniently downhole, in conjunction with orincorporated into a drilling mud, but alternative locations may also beselected. The core sample is then taken, in some non-limitingembodiments from a portion of the subterranean formation that isadjacent to a well-bore. The presence, or absence, of the tracertherein, and its quantification may then be determined by means ofanalyzing the fluid that is extracted from the sponge. Part of thisanalysis may, in this embodiment, include use of a retort distillationunit or Dean and Stark apparatus for removal of the water from thesponge, followed by hydrolysis thereof. Using appropriate timecalculations, as well as the measurement of the amount of light watertracer introduced, it is then possible to determine formation parameterssuch as porosity, permeability and fluids saturation. Other informationobtainable through use of this tracer include, but are not limited to,for example, identification of oil zones for secondary and tertiaryrecovery, oil/water contact zones, and transition zones.

It will be generally understood by those skilled in the art that if thelevel of deuterium or deuterium oxide at a specific location in thesponge (or in a specific discrete sponge liner) is found to be less thanwould be expected of formation or drilling mud-associated water alone,such as, in one embodiment, less than about 50 ppm, it will then beinferable that the introduced deuterium oxide-depleted water has enteredthe corresponding area of the core at that location. In other words, theformation will have been infiltrated or contaminated by the “filtrate”,which is whatever material carries the light water, e.g., a drillingmud, in combination with the light water tracer itself. If the amount ofdeuterium is found to be very low, such as, in another embodiment, lessthan about 25 ppm, it will be inferable that an even greater proportionof the injected light water has reached the core. Thus, filtratecontamination of the formation will have been confirmed.

Deuterium oxide-depleted water may also be used as a tracer in othertypes of coring, such as, in non-limiting embodiments, conventional,high torque, low invasion, high temperature/high pressure, horizontal,deepwater, gel, oriented, slimhole, and the like. It may also be usefulin a wide variety of other drillsite applications, such as, innon-limiting embodiments, fracturing, acidizing, cementing, fluid loss,depth control, field flood, and the like. It may additionally be usefulin association with equipment such as the Modular Formation DynamicsTester (MDT) (produced by Schlumberger), which tests and samples avariety of formation types to determine reservoir pressures,permeability, temperature, and the like, as well as with other types ofRepeat Formation Testing (RFT) methods. Those skilled in the art will beaware of the wide application of tracers in general and will understandthe benefits and advantages of substituting light water for certaintracers conventionally used in these and related applications. Nothingherein shall be construed as prohibiting simultaneous or sequential useof one or more additional tracers, such as, for example, tritium, forrelated or unrelated purposes.

The invention having been generally described hereinabove, those skilledin the art will appreciate that various modifications may be made withinthe scope of the invention, as defined by the claims appended hereto.Many potential embodiments may be envisioned by those skilled in theart, including, for example, application to a wide variety of drillsiteuses, involving a wide variety of types of equipment and methods ofdetection, quantification, analysis, transmission/receipt,interpretation, and the like, as well as uses of light water tracerhaving deuterium levels other than those explicitly disclosed herein.

The following examples are provided merely to further illustrate theinvention for the purpose of increasing the reader's overallunderstanding of it. As such they represent additional, butnon-limiting, potential embodiments of the invention.

EXAMPLE 1 Hypothetical

An amount of commercially purchased deuterium oxide-depleted water (2-3ppm deuterium) as a tracer is introduced, as a component of a drillingmud, into a subterranean formation via a well-bore. A sponge core sampleis then taken from the subterranean formation adjacent to the well-boreusing conventional sponge coring techniques and equipment. This sampleis taken at an appropriate point in time after introduction of thedeuterium oxide-depleted water, such that at least a portion of thedeuterium oxide-depleted water would likely have had time to migrate tothe region of the core sample, assuming such migration is possible. Thesponge core is then extracted to the surface, and the sponge linersappropriately preserved to ensure retention of any fluids absorbedtherein. The fluids are then removed from the sponge linerscorresponding to one specific core location by means of a Dean and Starkapparatus. The fluids are analyzed for the presence of deuterium usingconventional gas-chromatographic-mass spectrophotometric equipment andtechniques. From this analysis it is determined that the deuteriumconcentration of the fluid in the core at this location is only about 10ppm. From this it may be inferred that a large proportion of the tracerhas migrated to this location within the allocated time. From thisinformation the permeability of the formation may then be estimated.

1. A method of ascertaining or measuring an event comprising usingdeuterium oxide-depleted water as a tracer.
 2. The method of claim 1wherein the event being ascertained or measured occurs in a subterraneanformation.
 3. The method of claim 2 wherein the event is fluidmigration.
 4. The method of claim 3 wherein the fluid migrationindicates a property of a subterranean formation.
 5. The method of claim4 wherein the property is the porosity, permeability or fluidssaturation of the subterranean formation.
 6. The method of claim 5wherein the subterranean formation is being drilled for recovery of oilor gas.
 7. A method of ascertaining or measuring a subterranean eventcomprising transporting deuterium oxide-depleted water, as a tracer, toa first subterranean location; allowing a time period sufficient for thetracer to migrate to a second subterranean location, assuming suchmigration is possible; and using a tracer detection means or method tomeasure such tracer in the second subterranean location.
 8. The methodof claim 7 wherein the first subterranean location is a well-bore, thesecond subterranean location is a subterranean formation, and the tracerdetection means or method includes obtaining a core sample from thesubterranean formation.
 9. The method of claim 8 wherein the tracerdetection means or method is selected from the group consisting of massspectrophotometry, gas chromatography-mass spectrophotometry, HPLC massspectrophotometry, and nuclear magnetic resonance (NMR) imaging.
 10. Themethod of claim 8 wherein the core sample is a sponge core.
 11. Themethod of claim 7 wherein the second subterranean location is asubterranean formation and the measurement of tracer in the subterraneanformation is used to determine a property of the subterranean formation.12. The method of claim 11 wherein the property is porosity,permeability or fluids saturation.
 13. A method of ascertaining ormeasuring filtrate contamination of a subterranean formation comprisingtransporting deuterium oxide-depleted water, as a tracer, to a downholelocation having adjacent thereto a subterranean formation wherein fluidis located; allowing a time sufficient for the tracer to infiltrate theadjacent subterranean formation, assuming such infiltration is possible;obtaining a sample of the subterranean formation such that the fluidtherein is preserved; and measuring the amount of tracer in the fluid.14. The method of claim 13 wherein the sample of the subterraneanformation is a sponge core sample.
 15. The method of claim 14 furthercomprising extracting the fluid from the sponge core sample by means ofretort distillation or use of a Dean and Stark apparatus.
 16. The methodof claim 13 wherein a tracer detection means or method, selected fromthe group consisting of mass spectrophotometry, gas chromatograph-massspectrophotometry, HPLC mass spectrophotometry, and nuclear magneticresonance (NMR) imaging, is used to measure the amount of tracer in thefluid.
 17. The method of claim 13 wherein the amount of the tracer inthe fluid is used to determine a property of the subterranean formation.18. The method of claim 17 wherein the property is porosity,permeability or fluids saturation.
 19. The method of claim 13 furthercomprising transporting a second tracer.
 20. The method of claim 18wherein the second tracer is tritium.